Xiaolin Yu, Zixuan Zhou, Nikolaj A. Zhestkij, Yuliya Kenzhebayeva, Sviatoslav Povarov, Jiabo Chen, D. I. Pavlov, Nikita Burzak, Aleksandra R. Knyazeva, Stanislav S. Lazarev, Vyacheslav A. Dyachuk, Andrei Potapov, Lining Sun, Valentin A. Milichko, Vladimir P. Fedin
Lanthanide-based metal–organic frameworks (Ln-MOFs) provide a structurally precise platform for upconversion luminescence engineering, yet practical applications are limited by insufficient structural stability, restricted color tunability, and the lack of excitation in the NIR-III window. Here, we report a MOF incorporating mixed-lanthanide Yb 3+, Tb 3+, and Eu 3+ ions that addresses these challenges. The optically transparent and periodic framework enforces a uniform spatial distribution of lanthanide centers and establishes stepwise interionic energy transfer pathways, while ether-containing organic linkers enhance photo and thermal stability. Under NIR-I (980 nm) excitation, the Ln-MOFs exhibit tunable multicolor upconversion emission with high hydrolytic stability and biocompatibility, enabling reliable in vivo imaging in zebrafish models. Remarkably, under NIR-III (1960 nm) excitation, a four-photon upconversion process is observed for the first time in Ln-MOFs. Together, these results expand the excitation landscape of Ln-MOFs from NIR-I to NIR-III and confirm the potential of structurally programmable Ln-MOFs for bioimaging and nonlinear photonic applications.